Hydrogen/oxygen fuel cells produce in principle only water as their reaction product and hence receive attention as electric power generating systems that produce little adverse effect on the Earth's environment. Among them, solid polymer electrolyte fuel cells, in particular, are greatly expected to come in practice since their power density has been raised with rapid progress of study in recent years.
Conventionally, such a solid polymer electrolyte fuel cell usually outputs power by the reaction between a fuel gas supplied to the anode side and an oxidant gas containing oxygen to the cathode side, respectively, of a membrane electrode assembly in which gas-diffusive electrodes each provided with a catalyst layer containing a catalyst and an ion exchange membrane are joined with each other. The following methods, for example, are known as membrane electrode assembly production methods.    (1) Method in which a catalyst is caused to deposit directly on an ion exchange membrane (JP-B-58-47471).    (2) Method in which gas-diffusive electrode sheets having a catalytic activity are formed and the electrode sheets are joined with an ion exchange membrane (U.S. Pat. No. 3,134,697, U.S. Pat. No. 3,297,484, and JP-B-2-7398).    (3) Method in which two sets (half cells) of an ion exchange membrane and a catalyst layer formed thereon are formed and the two sets are adhered by pressure to each other with their respective ion exchange membrane sides facing each other to form a membrane electrode assembly.
Recently, the method (2) has been mainly employed in view of its merit that a small amount of a catalyst can be utilized effectively. The following processes, for example, have been proposed as specific processes for method (2). (2-1) Electrochemical deposition process (U.S. Pat. No. 5,084,144). (2-2) Process in which a coating solution containing a catalyst is applied onto an ion exchange membrane, or process in which a catalyst layer is formed by applying a coating solution containing a catalyst onto a gas diffusion layer, that is disposed adjacently to each catalyst layer to assist the catalyst layer in ensuring the stable gas-diffusibility thereof and to function also as a current collector, to obtain a electrode and two of the electrodes and an ion exchange membrane are joined together by means of a hot press or the like (coating process, JP-A-4-162365). (2-3) Process in which a catalyst layer is formed on a separately-provided base film, the catalyst layer is laminated with an ion exchange film, and the catalyst layer is transferred to the ion exchange membrane by hot-pressing (transfer process).
Also, method (3) has been tried since it has the merit of enabling the thickness of an ion exchange membrane to be reduced (JP-A-6-44984, JP-A-7-176317, and the like).
With the prior art transfer process noted above, however, it is required that the hot-pressing transfer be performed under such a low-pressure condition as not to crush a large number of fine pores which are present in the catalyst layers in order to ensure the gas-permeability within the catalyst layers. It is, therefore, difficult to completely transfer the catalyst layers to the membrane, resulting in a low yield or a high probability that the thickness of each catalyst layer becomes non-uniform. For this reason, a problem arises that it is difficult to adjust (make uniform) the amount of the catalyst in the plane direction of the membrane electrode assembly, hence, to obtain stable cell performance.
The coating process noted above conventionally employs a process of applying mainly a coating solution onto each gas diffusion layer in order to ensure fine pores within the catalyst layer, improve the gas-permeability and prevent concentration polarization in a high current density region. However, since such a gas diffusion layer is usually composed of porous carbon paper or carbon felt, a portion of uneven surface of the gas diffusion layer sometimes bites into the ion exchange membrane when the gas diffusion layer is joined with the ion exchange membrane by means of a hot press. In this case, the thickness of the ion exchange membrane partially varies and hence becomes non-uniform, raising problems including a lowered open circuit voltage due to gas leakage, short-circuit and the like. Thus, this process has a difficulty in stably producing membrane electrode assemblies using a thin ion exchange membrane having a thickness of not more than 30 im, for example.